1. Field of the Invention
The present invention relates generally to disk media.
2. State of the Art
Magnetic disk drive assemblies as used for mass data storage in computers and electronic systems today comprise either rigid ("hard") disk drives or flexible ("floppy") drives. Both types of drives incorporate low cost data storage capacity with rapid recovery of stored data. This rapid availability of stored data is a function of the rotational speed of the disk relative to the read/write transducer as well as the proximity of the transducer to the magnetic medium. In practice, a read/write transducer is mounted in a head assembly that accurately follows the surface of a disk at flying heights of less than 1 micron. In particular, the head suspension assemblies are designed to prevent contact between the read/write head and the magnetic recording medium during operation; such contact, called head crash, can destroy a read/write head and the magnetic medium in a short time due to the friction that results from the high rotational speed of the disk relative to the head. Although current technology provides lubrication and protective layers on the disk, these measures are generally intended to compensate only for transient friction events during stop/start cycles.
In general, control of the texture characteristics of the disk surface is required to reduce the substantial attractive forces that are generated between the read/write head and the stationary disk surface. Smoother disk surface textures result in higher attractive forces that prevent proper head liftoff and flying characteristics when disk rotation is commenced. Current disk manufacturing techniques must also assure that the disk surface roughness does not exceed certain upper-bound values; if excessive surface roughness results from the texturing process, undesirable increases in flying height also limit the density with which data can be stored on the disk. A central issue in current disk texturing processes is the reliability and consistency with which the desired surface roughness is obtained. The disk surface texture is typically characterized in terms of an arithmetic average roughness value (R.sub.a). Current disk texturing processes generally produce R.sub.a values in the range of 10-200 nm; the most modern disk drives achieve head flying heights of 0.2-0.3 microns with R.sub.a values of approximately 10-50 nm. These texturing processes utilize special abrasives for producing circumferential patterns of scratches on the surface of metallic (predominantly aluminum) disk substrates which inevitably create surface feature extremes in the form of peaks and valleys. U.S. Pat. Nos. 4,996,622, 4,939,614 and 4,931,338 describe variations of this general process. Several of these patents propose different textures for separate areas of the disk optimized for stop/start operations and for read/write operations. These patents document the difficulty of obtaining low flying heights (i.e., less than 0.3 microns) while simultaneously achieving acceptably low head/disk attractive forces with current disk texturing processes.
Other texturing processes combine abrasive texturing processes with chemical processes. For example, U.S. Pat. No. 4,985,306 describes a recording disk produced by subjecting a base plate containing S.sub.1 O.sub.2 --Li.sub.2 O--Al.sub.2 O.sub.3 series crystallized glass to crystallizing treatment, polishing the surface of the base plate to attain a surface roughness of 15 .ANG. to 50 .ANG. to evenly distribute, regularly and two-dimensionally, very fine and uniform crystal grains in the amorphous layer. The base plate is then etched with an etchant having different degrees of dissolution with respect to the crystal grains and the amorphous layer to form uniform and regular convexities and concavities on the surface of the base plate. A magnetic film and a protective layer are applied over the base plate. Because the system described in this patent relies on an abrasive texturing process for distributing crystal grains, there is an inevitable randomness to the ultimate distribution of concavities and convexities.
The trend toward smaller diameter disks has also presented difficulties for prior-art manufacturing techniques. It has become progressively more difficult to achieve the required consistency in R.sub.a values and in disk flatness with decreasing disk diameter using conventional methods. Disk flatness variations cause axial runout of the read/write head during disk rotation. In current disk drives it is desirable to maintain this axial runout value at less than 1-2 microns. Conventional abrasive texturing techniques applied to current metallic disk substrates are becoming less viable as disk diameters are progressing downwards.
The trend toward smaller diameter, higher density disks has made optical information storage technology a more attractive alternative for the future. Optical disks are typically made by first coating a thin layer of photosensitive material such as a thin metal film on one side of a glass disk. A laser beam is chopped by an electro-optic modulator to which a frequency modulated digital signal corresponding to the input information, such as an analog signal, is applied. The laser beam is focused onto the disk as it is rotated and the information is recorded as a series of pits in the thin metal film.
Reading of optical disks can be accomplished in several ways. For example, in a Video High Density Disk System (VHD), reading is accomplished by an electrode on a stylus that slides along the surface of the disk. Signals recorded on the disk are picked up as capacitance variations between the disk surface and the electrode on the stylus. In other systems, however, such as Video Long Play Systems, a low-power laser beam is focused on a small read spot on the surface of the disk. Optical energy is reflected by (or transmitted through) the disk and directed to a photodetector. The energy received at the photodetector changes according to the presence or absence of pits recorded on the disk. The received energy is processed by further electronic circuitry, such as by being processed into digitized form, and then into output signals.
While optical disks offer the advantage of recording large amounts of information in a small space, optical information storage technology suffers from certain disadvantages. For example, since information is normally stored on an optical disk by the heat of a laser beam causing the thin metal film over the substrate to develop pits, optical disks suffer the disadvantage of not being inherently erasable. Further, the technique of forming pits in the thin metal film by means of a laser involves heating the film to a higher temperature to affect the grain structure of the film. Bit errors resulting from factors such as changes in ambient temperature are common and have been a primary factor in limiting commercial application of optical technology beyond fields in which bit error rate is not highly critical. Accordingly, it is desirable to provide an information storage system coupling the advantages of optical systems, such as high density recording, with an erasable medium that is not subject to error due to changes in ambient temperatures.